In an apparent plea deal, Houghton denied a third charge, of theft, which was accepted by the prosecution.

The 25-year-old, who holds joint British-Dutch nationality and a computer science degree, was arrested leaving a central London hotel in March, carrying a briefcase that contained £900,000 cash. He had just met undercover MI5 officers, who he believed had bought British secrets on behalf of a foreign government.

Prosecutors disclosed in a previous hearing that MI5 counter-espionage investigators acted on a tip-off from Dutch authorities, who he had approached asking for £2m.

He had left MI6 after less than two years, apparently with Top Secret papers, and a laptop hard drive and memory sticks stuffed with highly classified information. It was ten months between his departure from the service and the climax of the operation to catch him.

Houghton pleaded guilty today to disclosing details of intelligence-gathering techniques, and disclosing two lists of British intelligence personnel. The theft charge, effectively abandoned today, accused him of stealing files from MI5.

The apparent bargain means details of how Houghton was able to leave MI6 with a cache of printed and electronic Top Secret material - which remained undetected until Dutch authorities alerted MI5 - will remain a worrying mystery. He will be sentenced on 3 September. ®

Update

According to The Guardian, Houghton had taken many of the files home on CDs and DVDs he was able to burn at the office. Which is reassuring.

It also reports his lawyers submitted psychiatric reports in mitigation claiming he had been "directed by voices".